Test if empty touches mesh/depth object with python
Thanks to the link of batFINGER to the function closest_point_on_mesh() I wrote a function which does the trick:

def is_empty_on_mesh (empty,mesh,maxDist = .000001):

    empty = bpy.data.objects[empty]
    mesh = bpy.data.objects[mesh]    

    # to compute distance via closest_point_on_mesh() the
    # objects have to be translated from global to local space
    emw = empty.matrix_world.to_translation()
    mmwi = mesh.matrix_world.inverted()

    local_pos = mmwi * emw
    (hit, loc, norm, face_index) = mesh.closest_point_on_mesh(local_pos)
    vec = mesh.matrix_world * loc

    dist = (vec - emw).length
    #print("Function Dist:",dist) # Debugging
    return(dist &lt; maxDist)

is_empty_on_mesh(NameOfEmpty, NameOfMesh)


The function returns TRUE or FALSE, depending on the distance being smaller than .000001 (this value is somehow arbitrary).